The following statement is a full description of the invention and includes the best method of performing it.
This invention relates medical diagnostic equipment and methods and is particularly concerned measuring intra-abdominal pressure using direct or remote sensing of pressure within the organ in particular Intra-abdominal pressure and related pressure within adjacent organs.
Until the advent of recent publication in relating to renal failure and complication of pressure after surgery few considered intra-abdominal pressure measurement to be important. It is now recognized as an important part of post-operative care.
Currently intra-abdominal pressure is measured using a urinary catheter requiring insertion of an extra T-piece or a needle directly into the urinary catheter to allow the pressure to be measured using a transducer or a manometer.
A wide variety of innovative techniques have been used to measure IAP in nearly every part of the abdominal cavity, including the rectum, stomach, urinary bladder, uterus, liver, inferior vena cava, and free within the intra-peritoneal cavity.
Rectal pressure measurement was experimentally popular in the early part of this century, using a Miller Abbott tube. It would appear generally it is a simple but slightly unreliable technique.
McCarthy in 1982 in a study of 12 patients undergoing urodynamic evaluation and laparoscopic tubal ligation found that there was a good correlation between intra-abdominal and rectal pressures. He expressed concern that reliability of this technique required the catheter to remain 10 cm above the anal verge otherwise the values were greater than the abdominal values. Presumably this was due to the spontaneous activity of the rectal sphincters. Shafik used rectal pressure in many of his experiments in humans on rectal detrusor muscle activity. He used a urinary catheter with an outer diameter of 1.2 mm and found that there was good visual correlation between intravesical and rectal pressures.
Intra-gastric measurement was used in the early part of this century with a Hamilton manometer, which afforded the simplest and most reliable technique at the time.
Simple techniques using nasogastric tubes to measure IAP have been used by Cullen, Fietsam and Collee. Concern has been expressed about simple perfusion techniques using a nasogastric tube, and Lacey in an animal study found that the use of gastric pressure measurement through an irrigational portal of the nasogastric tube is not reproducible. Collee, from London, used an unperfused nasogastric tube to obtain 141 paired measurements in 26 general surgery patients in ICU. He found using appropriate statistical modeling, that gastric pressure may be 2.5 cm of water above or below intra-vesical pressure.
The intra gastric route has two specific advantages. It can be used when there has been trauma to the bladder or where the patient does not have a urinary catheter in place. Gastric pressures are also very useful when there is a tense pelvic haematoma following pelvic trauma, as vesical pressures in this situation may not reflect general IAPs.
Direct cannulation of the peritoneal cavity had been used experimentally, but it is not as accurate as the intravesical technique and is invasive.
Motew used a Verres needle to measure IAP in an experiment on 12 women undergoing tubal ligation. The use of a Verres needle to measure IAP may not be accurate during flow states. It is also dependent on the degree of muscle relaxation required for the laparoscopy. Obeid and colleagues, from Detroit, reported in 1995 a comparison of LAP measurement using four techniques in 28 patients. These included an intra-gastric route via a simple NG, a laparoscopic insufflator rectal pressure via a modified oesophageal stethoscope and a standard intra-vesical method with a urinary catheter. Obeid found that with a standard 6 mmHg rise in IAP, as measured by the insufflator, this was best correlated with the intravesical measurements, with a rise of 5.7 mmHg (±9.8). The gastric and rectal pressures were less reliable with the following changes recorded, −0.7±9.8 mmHg and 3.3±8.8 mmHg respectively. He found the rectal and gastric pressures were more position dependent and less reliable than the intravesical approach. The specific limitation of the laparoscopic technique in Obeid's study is the lack of validation of the Stryker endoscopy high flow insufflator, which was used as the gold standard to compare with the other methods. In clinical practice pressures measured with such laparoscopic insufflators may fluctuate widely during surgery. This can be related to the depth of anaesthesia and port mechanics including blockage with blood or other products.
Because of the fluid dynamics in the abdominal cavity, IAP can also be measured through a central venous line if its tip is in the inferior vena cava. This has been utilized by a number of researchers. Lacey in a study of rabbits, comparing different sites of IAP measurement found an excellent correlation between IVC pressures and vesical LAP readings. It should be remembered that these experiments were performed in rabbits, under general anaesthesia.
In addition Lacey found that there was poor correlation between superior vena cava, rectus abdominus and rectal pressure.
The gold standard for IAP measurement has been the intravesical technique. Unfortunately Kron did not test the reliability of his technique and validation of the intravesical technique was undertaken and published by Iberti and colleagues at Mount Sinai medical centre in 1989. In a study of post-operative patients with closed intra-abdominal drains they compared urinary catheter measurements with those recorded from the abdominal drains. They used the pubis as the zero point which may give rise to slightly reduced as it lies above the mid point of the abdominal cavity.
Iberti's investigations revealed a good correlation between intra-abdominal and intra-vesical pressure. In addition he found that there was little effect of positive end expiratory pressure (PEEP) on IAP. I have modified the technique slightly and the technique used in this project is according to the protocol below;
Other techniques, including installation of saline into the bladder and holding the catheter in the air have also been described. They are cumbersome, do not provide on-line monitoring or are time consuming.
Previously the direct on-line monitoring of urinary catheters has not been reported as a measure of intra-abdominal pressure. Urinary catheters usually contain two lumens, one for the balloon and one for the urine flow.
For patients with haematuria, triple lumen catheters have been used for years. They allow irrigation through the third lumen. They have not been used or reported to measure intra-abdominal pressure.